riscv: cleanup Canaan Kendryte K210 sysctl driver

Introduce the header file include/soc/canaan/k210-sysctl.h to have a
common definition of the Canaan Kendryte K210 SoC system controller
registers. Simplify the k210 system controller driver code by removing
unused register bits definition. The MAINTAINERS file is updated,
adding the entry "CANAAN/KENDRYTE K210 SOC SYSTEM CONTROLLER DRIVER"
with myself listed as maintainer for this driver.
This is a preparatory patch for introducing the K210 clock driver. No
functional changes are introduced.

Signed-off-by: Damien Le Moal <damien.lemoal@wdc.com>
Signed-off-by: Palmer Dabbelt <palmerdabbelt@google.com>